Abstract

Binary gratings were fabricated with high first-order diffraction efficiency by conventional electron-beam drawing and subsequent inductive coupled-plasma dry etching upon the surfaces of SiO2 glass plates. The gratings were covered with a thin SiO2 film by plasma-enhanced chemical-vapor deposition without the grooves’ being filled in. The buried gratings exhibited first-order diffraction efficiencies of 84% for transverse-electric and 87% for transverse-magnetic polarized light at a wavelength of 1.55 μm when the period and the depth were 1.5 and 2.8 μm, respectively.

First-order transmission diffraction efficiencies of the grating for TE- and TM-polarized light, denoted by open and filled circles, respectively, in the wavelength region 1.5–1.58 μm. Each point includes the reflection losses at the flat back surface. The solid curve (TE) and the dashed curve (TM) are calculated efficiencies, including reflection losses, from the estimated dimensions of the cross section (Fig.
2). The incident angle coincides with the first-order reflective diffraction, which was approximately 30°. Inset, image of the measurement.

Cross-sectional SEM views of gratings with the groove depth D = 3 ± 0.1 μm before and after the 18-μm-thick overcladding. The period P and the groove width W of each grating are shown in the figure. The gratings with narrower grooves could be buried successfully.

Comparison of the measured and calculated first-order diffraction efficiencies of a grating buried in SiO2 for TE- and TM-polarized light. Open (TE) and filled (TM) circles show the measured efficiencies, and the solid (TE) and dashed (TM) curves show the calculated efficiencies. The reflection losses at the top and bottom surfaces are included in the measured and calculated values. The incident and diffracted beam angles are also shown.